The cell cycle of eukaryotic cells is controlled by cyclin-dependent kinases (cdks); these are kinases which require a regulatory subunit ("cyclin") in order to be active. Different processes in the cell cycle (such as replication and entry into mitosis) are controlled by different cdks (Morgan, Nature 374, 131 (1995)). In association with this, the activity of cyclin-dependent kinases is subject to a high degree of regulation. In this context, internal control mechanisms exist which, for example, prevent entry into mitosis until the DNA has completed its replication. Control by external factors, such as growth factors, only occurs before DNA replication begins; replication is initiated by active cyclin E/cdk2 complexes.
In addition to the quantity of cyclin in the kinase subunit, the activity of cyclin-dependent kinases is also regulated by small inhibitor proteins (Sherr and Roberts, Gene Dev. 9, 1149 (1995)). For example, two inhibitors, which are designated p21 and p27 in accordance with their size, are crucial for cyclin E/cdk2.
The cyclin E/cdk2 kinase is normally inactive in cells which are expressing high quantities of p27, and the entry into DNA replication is blocked.
While positive cell cycle regulators are overexpressed or at least expressed constitutively in many human tumors (Sherr, Science 274, 1672 (1996)), negative regulators are frequently mutated or only weakly expressed (Fero et al., Cell 85, 733 (1996)). Specific correlations exist: for example, the cyclin D1 gene is found to be overexpressed in many neck tumors. The hope therefore exists that cyclin-dependent kinases, and their function, might be target structures in the search for novel, selective substances which have an antiproliferative effect.
The gene for the p27 protein has been known for some years (K. Polyak et al. Cell 78, 59-66 (1994)) and is available in Genbank [murine p27; accession number K 09968; human p27: K 10906]. Despite intensive searching, mutations in the p27 gene have not so far been found in human tumors. This is all the more surprising since mice in which the gene for p27 has been inactivated exhibit a phenotype with multiple dysplasias and an increased incidence of tumors (Fero et al., Cell 85, 733 (1996), Kiyokawa et al., Cell 85, 721 (1996)). Instead of this, the function of p27 is evidently in the main regulated posttranscriptionally.
Thus, p27 is degraded by proteolysis; this also occurs at the beginning of DNA replication in normal cells. The ability to degrade p27 proteolytically is markedly increased in many tumors, as compared with the normal tissue, and this appears to correlate directly with an unfavorable prognosis (Loda et al., Nature Med. 3, 231 (1997)).
However, there must also be other mechanisms as well which are able to lead to p27 inactivation. For example, after cells have been transformed with the Myc oncogene, p27 is first of all inactivated functionally (i.e. it no longer binds to cyclin/cdk complexes) and is only degraded at a much later stage. It has not so far been possible to understand why large quantities of p27 proteins are expressed in a number of breast tumors, for example, and these tumors nevertheless grow very rapidly. The mechanisms which inactivate p27 in these tumors are so far unknown (Fredersdorf et al., Proc. Natl. Acad. Sci. USA 94, 6380 (1997)).
There is consequently a great interest among experts in finding mechanisms or substances which are responsible for inactivating p27 in tumors. The present invention has solved this problem. The protein p163, which is described in the application, can bind p27, can inhibit the function of p27 and can lead p27 to proteolysis in the cytoplasm.